Doc: Replace the example for QFileInfo::setFile

The example in the documentation of `QFileInfo::setFile` made no use
of `setFile` and only showed a use of `QDir::setCurrent`.

The example was replaced with a new example showing how `setFile`
changes the file that the information are retrieved from.

The old example was moved under the documentation for
`QDir::setCurrent` as it shows its working.

Fixes: QTBUG-87128
Change-Id: I8227876cfcb4d582040bda9b4b7f3f7debea1e07
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit a2abb0145174a8ed82572a06e537f550a6777b08)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/snippets/code/src_corelib_io_qdir.cpp

@@ -182,4 +182,17 @@ namespace MyNamespace
 Q_CLEANUP_RESOURCE(myapp);
 //! [15]
 
+//! [16]
+QString absolute = "/local/bin";
+QString relative = "local/bin";
+QFileInfo absFile(absolute);
+QFileInfo relFile(relative);
+
+QDir::setCurrent(QDir::rootPath());
+// absFile and relFile now point to the same file
+
+QDir::setCurrent("/tmp");
+// absFile now points to "/local/bin",
+// while relFile points to "/tmp/local/bin"
+//! [16]
 }

src/corelib/doc/snippets/code/src_corelib_io_qfileinfo.cpp

@@ -90,20 +90,17 @@ info2.size();               // returns 63942
 
 
 //! [2]
-QString absolute = "/local/bin";
-QString relative = "local/bin";
-QFileInfo absFile(absolute);
-QFileInfo relFile(relative);
+QFileInfo info("/usr/bin/env");
 
-QDir::setCurrent(QDir::rootPath());
-// absFile and relFile now point to the same file
+QString path = info.absolutePath(); // path = /usr/bin
+QString base = info.baseName(); // base = env
 
-QDir::setCurrent("/tmp");
-// absFile now points to "/local/bin",
-// while relFile points to "/tmp/local/bin"
+info.setFile("/etc/hosts");
+
+path = info.absolutePath(); // path = /etc
+base = info.baseName(); // base = hosts
 //! [2]
 
-
 //! [3]
 QFileInfo fi("/tmp/archive.tar.gz");
 QString name = fi.fileName();                // name = "archive.tar.gz"

src/corelib/io/qdir.cpp

@@ -1951,6 +1951,8 @@ QFileInfoList QDir::drives()
     Returns \c true if the directory was successfully changed; otherwise
     returns \c false.
 
+    \snippet code/src_corelib_io_qdir.cpp 16
+
     \sa current(), currentPath(), home(), root(), temp()
 */
 bool QDir::setCurrent(const QString &path)